The present invention relates to a process for producing a polyester resin composition suitable for use as a starting material for the production of a white film. More particularly, the present invention relates to a process for producing a polyester resin composition of high quality having a high concentration (.ltoreq.10 wt %) of a white inorganic pigment dispersed uniformly in a polyester resin.
White inorganic pigments typified by titanium dioxide are conventionally dispersed in polyester resins typified by polyethylene terephthalate (PET) by either one of the following methods: adding such pigments per se; or suspending them in an ester-forming polyhydric alcohol such as ethylene glycol to prepare a slurry and adding the slurry during the process of ester exchange or esterification reactions and before polycondensation. The latter method is disclosed in many prior patents such as Japanese Patent Publication Nos. 945/1958, and 18135/1981. But this method has the problem that the particles of white inorganic pigment added tend to agglomerate and precipitate in the resulting polymer, and the amount of pigment that can be incorporated is no more than several percent.
If one wants to incorporate a fairly large amount (.ltoreq.10 wt %) of white inorganic pigment in a polyester resin and make a uniform dispersion, the viscosity of the polyester resin must be increased and high shear force must be applied. To this end, a twin-screw kneading extruder is usually employed and in order to knead the white inorganic pigment for producing a uniform dispersion, the rotational speeds of the screws and the residence time must be increased. However, if kneading operations are performed under these conditions, the high shear force applied generates heat and the resulting resin composition suffers from such troubles as reduction in the degree of polymerization, coloration and formation of decomposition products. Thermal deterioration occurring in polyester resins exposed to the conditions described above is particularly estensive and has caused serious problems in association with the quality of the final product.
With a view to solving these problems, Unexamined Published Japanese Patent Application (hereinafter referred to as OPI) No. 250034/1986 proposes a method that comprises mixing a polyester resin with a pigment in molten state to prepare a master batch, subjecting the master batch to solid-phase polymerization, and mixing the resulting polymer with another feed of the polyester resin. In this method, the step of solid-phase polymerization is continued for as long as 5-20 hours in order to increase the degree of polymerization that has dropped on account of thermal deterioration and to remove the by-products resulting from decomposition. However, this method still suffers from such disadvantages as increased running costs and inadequacy in the quality of the finally obtained resin composition.
Japanese Patent Publication No. 17664/1973 discloses a method for preventing the decrease in the molecular weight of a resin by kneading it with a twin-screw kneader or a roller kneader placed under vacuum, an inert gas stream or some other condition that is isolated from the atmosphere. This is a good method for preventing the decrease in the molecular weight of a resin to be mixed with a pigment. However, in order to attain a sufficiently wide surface area in a twin-screw kneader to provide an increased molecular weight under vacuum, a certain space is necessary between the barrel and each screw but then the shear foce produced between the barrel and the screws is reduced or the "short pass" of the molten polymer causes insufficient dispersion of the pigment particles in the polymer. For these reasons, the method described in Japanese Patent Publication No. 17664/1973 is not suitable for the purpose of kneading pigments that have a great tendency for agglomeration. With a roller kneader, the clearance between rollers must be precisely maintained at about several tens of microns. However, if the throughput is several hundreds of kilograms per hour, it is extremely difficult to keep this level of precision in the temperature range set forth in said patent unless very expensive equipment is employed.
Japanese Patent Application (OPI) No. 212908/1983 discloses a method in which a molten plastic is fed into a twin-screw kneader/extruder and kneaded with additives, followed by combining the blend with another supply of the molten plastic in the rear portion of the extruder so as to inhibit excessive heat generation while ensuring the necessary throughput. The problem with this method is that it does not have any means for restoring the resin from its thermal deterioration due to the heat generated during the kneading operation and that a comparatively high degree of kneading is necessary in order to attain a uniform dispersion after the blend of resin and additives is combined with another supply of the resin. To meet this need, either the residence time or the rotational speeds of the screws must be increased but then the generation of heat will take place again.